1. Field of the Invention
The present invention relates to semiconductor device fabrication and more particularly, to a reticle that improves the alignment or stacking accuracy in the lithographic process for fabricating semiconductor devices, and a method of fabricating a semiconductor device that uses a reticle improving the alignment or stacking accuracy in the lithographic process.
2. Description of the Related Art
The lithography technique, which is used to transfer patterns of geometric shapes for on a mask to a thin resist layer, plays an important role in the process sequence of fabricating Ultra-Large Scale Integrated semiconductor devices (ULSIs). To conduct the lithographic process adequately, there is the need to form minute patterns of geometric shapes on a reticle (hereinafter, reticle patterns) as correct as possible and to align or stack the reticle patterns to the patterns of an underlying layer at as high accuracy as possible.
With the reduction step-and-repeat projection exposure system (which is simply called the xe2x80x9cstepperxe2x80x9d), which has been usually used for the optical lithographic process in the ULSI fabrication process sequence, the optical lenses have some aberration and therefore, an image of the reticle patterns transferred to a resist layer on the semiconductor wafer has image distortion and positional distortion. The amount of the image and positional distortions varies dependent on the size and pitch of the reticle patterns.
To meet the above-described need to form the reticle patterns as correct as possible and to align the reticle patterns to the underlying layer at as high accuracy as possible, an improved reticle was developed and disclosed in the Japanese Non-Examined Patent Publication No. 10-213895 published in August 1998. The improved reticle includes alignment marks, each of which is formed by the combination of geometric shapes having the same size and the same shape as the geometric shapes that form the circuit pattern in each chip site of the wafer. For example, if the reticle is designed for circular or square contact holes, the alignment mark is formed by the combination of circular or square shapes having the same size as the circular or square contact holes. The circular or square shapes are arranged regularly to define the contour of the alignment mark.
With the improved reticle disclosed in the Publication No. 10-213895, the circuit patterns formed in the chip area and the alignment mark pattern formed in the scribe line area have a specific geometric correlation and thus, the amount of the image and positional distortions generated in the chip area is substantially equal to the scribe line area. Accordingly, the alignment or stacking accuracy of the circuit patterns in the chip area is correctly recognizable by measuring the alignment or stacking accuracy of the alignment mark. This makes it possible to cope with further miniaturization of circuit patterns in the optical lithographic process.
Next, the alignment mark of the improved reticle disclosed in the Publication No. 10-213895 is explained in more detail below with reference to FIG. 1.
As seen from FIG. 1, the alignment mark 500 of the prior-art reticle comprises four rectangular mark elements 501A, 501B, 503A, and 503B, which are arranged to form an imaginary rectangle in the scribe line area of the reticle. The two elements 501A and 501B, which extend along the x-axis, are parallel to and apart from each other by a specific distance. The two elements 503A and 503B, which extend along the Y-axis perpendicular to the X-axis, are parallel to and apart from each other by a specific distance.
The prior-art reticle with the mark 500 is applied to the upper one of the two adjoining layers. Another alignment mark 600 due to another reticle, which is additionally shown by broken lines in FIG. 1, is formed on the lower one of the two layers. This is to exhibit the positional relationship between these marks 500 and 600.
Similar to the upper-layer mark 500, the lower-layer mark 600 comprises four rectangular mark elements 605A, 605B, 607A, and 607B, which are arranged to form an imaginary rectangle smaller than that of the mark 500 in the scribe line area. The two elements 605A and 605B, which extend along the X-axis, are parallel and apart from each other by a specific distance. The two elements 607A and 607B, which extend along the Y-axis perpendicular to the X-axis, are parallel and apart from each other by a specific distance. As shown in FIG. 1, the mark 500 is located to surround entirely the mark 600.
FIG. 2 shows the detailed structure of the part 509 of the mark element 503B of the mark 500 shown in FIG. 1. As shown in FIG. 2, the part 509 of the element 503B includes a pattern 510 comprising seven linear sub-elements 511, 512, 513, 514, 515, 516, and 517 with the same size and the same shape. These sub-elements 511 to 517 are formed to extend parallel to the Y-axis and are arranged along the X-axis at equal pitches p. The pitch p is equal to the pitch of the linear sub-elements of the circuit pattern formed in the chip area of the reticle. A plurality of the patterns 510 are arranged at regular intervals along the Y-axis, thereby forming the element 503B.
The mark element 503A has the same structure as the element 503B shown in FIG. 2. The alignment mark elements 501A and 501B have the structure obtained by turning the element 503B by 90xc2x0 around the center of the imaginary rectangle of the mark 500.
When the prior-art reticle having the alignment mark 500 is used in the optical lithographic process, the following pattern is formed on an optical resist layer over a semiconductor wafer. The pattern formed in the resist layer is termed the xe2x80x9cresist patternxe2x80x9d in the following explanation.
FIG. 3 shows an example of the resist pattern 520 obtained from the pattern 510 of the element 503B of the prior-art alignment mark 500, which is formed by conducting the optical lithographic process using an ideal optical system of a so-called stepper without any aberration.
As shown in FIG. 3, the resist pattern 520 comprises seven linear sub-elements 521, 522, 523, 524, 525, 526, and 527 corresponding to the seven linear sub-elements 511, 512, 513, 514, 515, 516, and 517 of the pattern 510 of the element 503B. Since it is supposed that the optical system of the stepper includes no aberration, as shown in FIG. 3, the centerlines CL21, CL22, CL23, CL24, CL25, CL26, and CL27 of the sub-elements 521 to 527 are respectively located on their specific reference positions. In other words, the centerlines CL21 to CL27 of the sub-elements 521 to 527 have no positional shift with respect to their reference positions.
FIG. 4 shows an example of the resist pattern 530 obtained from the pattern 510 of the element 503B of the prior-art alignment mark 500, which is formed by conducting the optical lithographic process using an actual optical system of a so-called stepper with aberration.
As shown in FIG. 4, the resist pattern 530 comprises seven linear sub-elements 531, 532, 533, 534, 535, 536, and 537 corresponding to the seven linear sub-elements 511, 512, 513, 514, 515, 516, and 517 of the pattern 510 of the element 503B.
Since the optical system of the stepper includes some aberration, as shown in FIG. 4, the sub-elements 531 to 537 have positional shifts with respect to their reference positions (i.e., the sub-elements 521 to 527 in FIG. 3), respectively. Specifically, the centerlines CL31, CL32, CL33, CL34, CL35, CL36, and CL37 of the sub-elements 531 to 537 are respectively deviated from the centerlines CL21, CL22, CL23, CL24, CL25, CL26, and CL27 located respectively on their reference positions by specific shifts C, D, E, F, G, H, and I. The shifts C and I of the sub-elements 531 and 537 located at the right and left edges are larger than the shifts D, E, F, G, and H of the shapes 532, 533, 534, 535, and 536 located inwardly.
The difference between the shifts C and I and the shifts D, E, F, G, and H is caused by the diffraction level difference of the irradiated light. This is due to the fact that the sub-elements 511 and 517 of the pattern 510 of the alignment mark 500 are half-isolated (i.e., only the sub-elements 512 and 516 are respectively placed adjacent to the sub-elements 511 and 517) while the sub-elements 512, 513, 514, 515, and 516 are not isolated (i.e., two sub-elements are respectively placed at both side of each sub-element 512, 513, 514, 515, or 516). Thus, the optical images corresponding to the sub-elements 511 and 517 have larger shifts than those corresponding to the sub-elements 512 to 516. As a result, the shifts C and I of the sub-elements 531 and 537 of the resist pattern 530 are larger than the shifts D, E, F, G, and H of the sub-elements 532 to 536. In other words, the spacial frequency characteristic of the sub-elements 531 and 537 located at the edges of the resist pattern 530 is different from that of the sub-elements 532 to 536 located inside the resist pattern 530.
Generally speaking, coma aberration of an optical system varies dependent on its spacial frequency characteristic. Accordingly, with the above-described prior-art reticle, due to the difference of the spacial frequency characteristic, the alignment or stacking accuracy of the circuit pattern formed in the chip area is unequal to that of the alignment mark 500 formed in the scribe line area, resulting in a problem of alignment or stacking error of the circuit pattern.
Specifically, if the sub-elements 511 to 517 of the pattern 510 of the alignment mark 500 produce the resist pattern 530 with the linear sub-elements 531 to 537 shown in FIG. 5A, the linear sub-elements 531 to 537 of the resist pattern 530 result in the reflected-light intensity distribution shown in FIG. 5B. In this case, the alignment or staking accuracy of the circuit pattern on the reticle with respect to the circuit pattern of the underlying layer is measured by recognizing or detecting the edges 531a and 537a of the sub-elements 531 and 537 located at the edges of the resist pattern 530 from the reflected-light intensity distribution of FIG. 5B. Accordingly, if the spacial frequency characteristic of the sub-elements 531 and 537 is different from that of the inner sub-elements 532 to 536, the measurement result using the alignment mark 500 located in the scribe line area of the reticle reflects incorrectly the alignment or staking accuracy of the circuit pattern located in the chip area of the same reticle. This incorrectness will degrade the alignment or stacking accuracy of the optical lithographic process itself.
Because of the above-described reason, it is desirable to provide a reticle with an alignment mark that eliminates the spacial frequency characteristic error or difference of the sub-elements of the resist pattern, thereby reducing the incorrectness in the measurement result about the alignment accuracy of the circuit pattern.
Also, it is desirable to provide an improved method of fabricating a semiconductor device that equalizes the spacial frequency characteristic of the alignment mark pattern formed in the scribe line area of the reticle to that of the circuit pattern formed in the chip area of the same reticle.
Accordingly, an object of the present invention is to provide a reticle and a method of fabricating a semiconductor device that improve the alignment or stacking accuracy in the optical lithographic process.
Another object of the present invention is to provide a reticle and a method of fabricating a semiconductor device that reduce the error or incorrectness of alignment accuracy measurement using the alignment mark in the optical lithographic process.
The above objects together with others not specifically mentioned will become clear to those skilled in the art from the following description.
According to a first aspect of the present invention, a reticle is provided, which comprises a first area including a desired circuit pattern and a second area including alignment marks arranged at specific positions, the first area and the second area being located in an exposure range of an optical exposure apparatus.
Each of the alignment marks comprises mark elements arranged to form a first geometric shape.
Each of the mark elements has main sub-elements arranged in a specific direction at first pitches to from a second geometric shape, a first auxiliary sub-element located at one end of the second geometric shape, a second auxiliary sub-element located at the other end of the second geometric shape. The first auxiliary sub-element is apart from a first one of the main sub-elements at a second pitch. The second auxiliary sub-element is apart from a second one of the main sub-elements at a third pitch.
Each of the main sub-elements is resolvable in the apparatus. Each of the first and second auxiliary sub-elements is irresolvable in the apparatus.
With the reticle according to the first aspect of the present invention, each of the alignment marks, which are formed in the second area, comprises mark elements arranged to form a first geometric shape. Each of the mark elements has main sub-elements arranged in a specific direction at first pitches to from a second geometric shape, a first auxiliary sub-element located at one end of the second geometric shape, a second auxiliary sub-element located at the other end of the second geometric shape. The first auxiliary sub-element is apart from a first one of the main sub-elements at a second pitch. The second auxiliary sub-element is apart from a second one of the main sub-elements at a third pitch. Each of the main sub-elements is resolvable in the apparatus. Each of the first and second auxiliary sub-elements is irresolvable in the apparatus.
Accordingly, when specific light is irradiated to a resist layer through the reticle using the optical exposure apparatus, the first and second auxiliary sub-elements of each alignment mark do not form any optical images in the resist layer. On the other hand, the main sub-elements of each alignment mark form optical images with substantially equal aberration-induced shifts in the resist layer. This is because each of the main sub-elements is not isolated due to existence of the first and second auxiliary sub-elements.
As a result, only the main sub-elements of each alignment mark can be selectively transferred to the resist layer by the optical exposure apparatus without unequal aberration-induced shifts in the images of each alignment mark. In other words, all the images of the main sub-elements of each alignment mark in the resist layer exhibit a correct spacial frequency characteristic according to the first pitches of the main sub-elements when the alignment accuracy is measured.
Thus, the alignment accuracy is improved in the optical lithographic process using the reticle with the alignment mark. This means that the error or incorrectness of alignment accuracy measurement using the alignment marks in the optical lithographic process is reduced.
In a preferred embodiment of the reticle according to the first aspect, each of the main sub-elements has a linear shape, and each of the first and second auxiliary sub-elements has a linear shape. In this embodiment, there is an additional advantage that the main sub-elements and the first and second auxiliary sub-elements can be easily formed on the reticle at equal pitches to the circuit pattern in the first area and therefore, the alignment accuracy is further improved.
In another preferred embodiment of the reticle according to the first aspect, each of the main sub-elements has a linear shape with a width greater than a specific threshold width equal to an exposure limit of the apparatus. Each of the first and second auxiliary sub-elements has a linear shape with a width equal to or less than the specific threshold width. In this embodiment, along with the additional advantage that the alignment accuracy is further improved, there is another additional advantage that each of the main sub-elements can be easily made resolvable and each of the first and second auxiliary sub-elements can be easily made irresolvable in the apparatus.
In the above-identified preferred embodiments of the reticle according to the first aspect, the linear shape of each of the main sub-elements may be continuous or broken (or divided) between its two ends. Similarly, the linear shape of each of the first and second auxiliary sub-elements may be continuous or broken (or divided) between its two ends.
In still another preferred embodiment of the reticle according to the first aspect, the specific direction, in which the main sub-elements are arranged is a measuring direction in alignment accuracy measurement.
In a further preferred embodiment of the reticle according to the first aspect, the second pitch of the first auxiliary sub-element and the third pitch of the second auxiliary sub-element are approximately equal to the first pitch of the main sub-elements. In this embodiment, there is an additional advantage that the first and second auxiliary sub-elements can be easily formed on the reticle.
In a still further preferred embodiment of the reticle according to the first aspect, the first pitch of the main sub-elements is substantially equal to a pitch of the circuit pattern formed in the first area. In this embodiment, there is an additional advantage that the reproducibility of the alignment accuracy in the optical lithographic process is enhanced.
In a still further preferred embodiment of the reticle according to the first aspect, a third auxiliary sub-element and a fourth auxiliary sub-element are additionally provided to be arranged in the specific direction of the main sub-elements. The third auxiliary sub-element is located to be adjacent to the first auxiliary sub-element at a fourth pitch. The fourth auxiliary sub-element is located to be adjacent to the second auxiliary sub-element at a fifth pitch. In this embodiment, the above-described advantages of the reticle according to the first aspect are further enhanced.
Preferably, each of the third and fourth auxiliary sub-elements has a linear shape, or each of the third and fourth auxiliary sub-elements has a linear shape with a width equal to or less than the specific threshold width. The linear shape of each of the third and fourth auxiliary sub-elements may be continuous or broken (or divided) between its two ends.
According to a second aspect of the present invention, a method of fabricating a semiconductor device is provided, which comprises the steps of:
(a) forming an optical resist layer on a target layer located over a surface of a semiconductor wafer;
(b) irradiating light to the resist layer through a reticle comprising a first area including a desired circuit, pattern and a second area including alignment marks arranged at specific positions;
the first area and the second area being located in an exposure range of the light;
each of the alignment marks comprising mark elements arranged to form a first geometric shape;
each of the mark elements having sub-elements arranged in a specific direction at specific pitches to from a second geometric shape;
(c) developing the resist layer exposed to the light to form a resist circuit pattern and resist alignment-mark patterns in the resist layer from the circuit pattern and the alignment marks on the reticle, respectively;
each of the resist alignment-mark patterns including resist sub-patterns corresponding to the sub-elements arranged in the specific direction;
(d) selectively removing two ones of the resist sub-patterns located at their two ends from each of the resist alignment-mark patterns, forming amended resist alignment-mark patterns in the resist layer;
(e) measuring alignment accuracy of the resist layer with respect to an underlying layer of the target layer using the amended resist alignment-mark patterns; and
(f) selectively etching the target layer using the resist layer with the amended resist alignment-mark patterns.
With the method of a semiconductor device according to the second aspect of the present invention, the resist layer exposed to the light is developed in the step (c) to form a resist circuit pattern and resist alignment-mark patterns in the resist layer from the circuit pattern and the alignment marks on the reticle, respectively. Each of the resist alignment-mark patterns includes resist sub-patterns corresponding to the sub-elements arranged in the specific direction. Then, two ones of the resist sub-patterns located at their two ends are selectively removed from each of the resist alignment-mark patterns in the step (d), forming amended resist alignment-mark patterns in the resist layer. Thereafter, alignment accuracy of the resist layer with respect to an underlying layer of the target layer is measured using the amended resist alignment-mark patterns in the step (e).
Accordingly, it is said that the two ones of the resist sub-patterns located at their two ends, which are removed in the step (d), correspond to the first and second auxiliary sub-elements in the reticle according to the first aspect. Also, it is said that the remaining resist sub-patterns correspond to the main sub-elements in the reticle according to the first aspect. As a result, there are the same advantages as those in the reticle according to the first aspect.
In the method according to the second aspect, the prior-art reticle described previously or the reticle according to the first aspect may be used as the reticle in the step (b).
In a preferred embodiment of the method according to the second aspect, each of the sub-elements of the mark elements has a linear shape, which may be continuous or broken (or divided) between its two ends.
In another preferred embodiment of the method according to the second aspect, the specific direction in which the sub-elements of the reticle are arranged is a measuring direction in the step (e).
In still another preferred embodiment of the method according to the second aspect, the pitch of the sub-elements of the reticle is substantially equal to a pitch of the circuit pattern of the reticle.